Thermodynamics of the Solubility and Permeation of Hydrogen in Metals at High Temperature and Low Pressure

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Abstract
The solubility and permeation of hydrogen in metals is analyzed on the basis of classical thermodynamics plus a minimum of kinetic theory. Expressions are derived for x , the number of H atoms absorbed per solid atom, and J , the permeation rate, in the limiting cases of continuum and free‐molecule flow. For most conditions of temperature (T) and total pressure (p t = p H + p H 2 ) , the expressions reduce to the familiar equations for solubility and permeation, i.e., a curve for constant p t appears as a straight line on Arrhenius plots of log x vs 1 / T and of log J vs 1 / T . However, for extreme conditions of high temperature and low pressure, the expressions predict that a curve of constant p t will become increasingly nonlinear as T increases, and it may even pass through a maximum when T becomes sufficiently high to produce almost complete dissociation of the gaseous H2. Also, under certain conditions neither x nor J is proportional to the square root of the total pressure. The analysis provides a simple explanation of existing experimental data on the solubility and permeation of hydrogen in tungsten and in molybdenum at high temperature and low pressure.

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